1. Field of the Invention
The present invention concerns a device for recognizing information conveyed by a signal received via a transmission channel and therefore likely to be affected by distortion and/or interference by the very fact of its transmission via that channel.
The present invention can be applied to digital information, i.e. information represented by convention by elementary forms of the signal to be transmitted each of which is defined for a duration equal to that of an information symbol.
The present invention is applicable to the demodulation of signals modulated using discrete modulation (such as FSK, MSK or GMSK modulation) and received via a radio transmission channel, for example a mobile radio channel.
Thus the present invention is applicable to receiving signals transmitted in a GSM (Global System for Mobile communications) type mobile radio system.
2. Description of the Prior Art
A signal received after transmission through a radio channel is degraded by the following phenomena:
variable attenuation due to losses (depending on the distance between the transmitter and the receiver) and the presence of obstacles along the transmission path (hills, buildings, vegetation, etc), PA1 multipath propagation caused by reflection or diffraction of the transmitted signal by various obstacles, PA1 the addition of noise (such as thermal noise), and interference due to others using the same resources (co-channel interference) or nearby resources (adjacent channel interference). PA1 correlator means for establishing a correlation between the received signal and various possible forms of signal, in accordance with said convention, and PA1 a neural network using correlation coefficients obtained from said correlator means, said network being trained by application to its input of correlation coefficients corresponding to a received signal conveying given information whilst said given information is imposed at its output, said network supplying recognized information.
The Viterbi algorithm can be used for such demodulation (see, for example, pages 256-258 of "The GSM System for Mobile Communications" by Michel MOULY and Marie-Bernadette PAUTET). This algorithm reconstitutes the most likely sequence of symbols transmitted by making a number of hypotheses as to the possible signals and the noise characteristics.
The present invention concerns a device for recognizing information conveyed by a received signal that can be applied to the demodulation of signals conveying digital information received via a radio transmission channel, for example a mobile radio channel, this device being based on a different technique whereby such hypotheses can be dispensed with.
The present invention uses neural networks. The present invention is essentially directed to applying to a neural network used to recognize information conveyed by a received signal, rather than the received signal itself, parameters obtained from the signal that are the most pertinent to the task to be accomplished by the network and which therefore substantially reduce the complexity of the network and/or enhance the quality of recognition.
The GSM recommendations specify that the demodulator must be able to process a signal in the case of two equal power multipaths spaced by 16 .mu.s. This is regarded as a limiting situation given the level of intersymbol interference that is generated and in this case equalization is required beforehand, this equalization necessitating an estimate of the impulse response of the transmission channel.
Another aspect of the invention lies in the simplification resulting from the fact that the two operations of equalization and demodulation usually carried out separately and successively, for example in a GSM receiver, can now be carried out in a single operation and the fact that the estimate of the impulse response of the transmission channel usually required to carry out said equalization operation is no longer needed.
A receiver for phase modulated signals (for example a GSM receiver) requires two channels in quadrature (called the I and Q channels) in order to be able to demodulate the signals. Another aspect of the invention lies in a further simplification due to the fact that only one of the two quadrature (I and Q) channels usually required in a demodulator for phase modulated signals (as in the GSM system mentioned above, in particular) is needed, although this does not prevent the use of two quadrature channels, in particular to implement a form of reception diversity.